The present invention relates to a high-frequency circuit element, such as a filter, etc., used for a high-frequency signal processor in communication systems, etc.
In high-frequency communication systems, a high-frequency circuit element such as a filter, a branching filter, or the like, is an essential component. Many of conventional high-frequency circuit elements, such as resonator filters, etc. have a structure that is electromagnetically shielded by enclosing a resonator in a metal box in order to prevent the radiation of electromagnetic field energy and the entrance of electromagnetic noise from the outside.
FIGS. 7 and 8 respectively show one example of a conventional high-frequency circuit element using a transmission line structure. In FIG. 7, a box lid 81 shown in FIG. 8 is omitted so that the internal structure of the box of the high-frequency circuit element can be seen.
As shown in FIG. 8, in the conventional high-frequency circuit element, xcex/2 resonators 86a, 86b, 86c, 86d of a strip conductor pattern and input/output lines 87a, 87b are formed on the surface of the subtrate 85 made of dielectric monocrystal, or the like. The high-frequency circuit having a microstrip structure is fabricated from xcex/2 resonators 86a, 86b, 86c, 86d, input/output lines 87a, 87b and a ground plane 88. This high-frequency circuit includes four coupled xcex/2 resonators and functions as a four-stage band pass filter.
The box of the high-frequency circuit element is formed of a conductor material and includes, as shown in FIG. 8, a box frame 82(also see FIG. 7) and a box bottom 83(also see FIG. 7). A substrate 85(see FIG. 7) is fixed to the box bottom 83 with, for example, conductive adhesives so that the box is electrically connected to the ground plane 88.
On the side face of the box frame 82, input/output terminals 84a, 84b(also see FIG. 7) having a coaxial connector are placed. Inner conductors of the input/output terminals 84a, 84b are electrically connected to the input/output lines 87a, 87b and outer conductors of the input/output terminals 84a, 84b are electrically connected to the box, respectively.
FIGS. 9 and 10 respectively show another example of a conventional high-frequency circuit element using a strip conductor pattern. Also in FIG. 97, a box lid 81 shown in FIG. 10 is omitted so that the internal structure of the box of the high-frequency circuit element can be seen. In the configuration shown in FIG. 10, eight hairpin resonators 89a, 89b, 89c, 89d, 89e, 89f, 98g, 86h are used so as to form an eigth-stage band pass filter. The structure of the other parts is the same as the conventional high-frequency circuit element shown in FIGS. 7 and 8 and will not be further described.
When inputting a high-frequency signal from, for example, the input/output terminal 84a in the conventional high-frequency circuit elements having the structure mentioned above, as shown in FIG. 11, electromagnetic waves in a quasi-TEM mode, i.e. a fundamental mode propagate through the input/output line 87a that is a microstrip conductor transmission line. FIG. 11 is a cross-sectional view showing the box, which is cut along the plane perpendicular to the input/output line 87a. FIG. 11 schematically shows the directions of the electric fields based on the analysis results of a three-dimensional electromagnetic field simulation (HFSS manufactured by Hewlett Packard Company).
The electromagnetic waves in the quasi-TEM mode propagating though the input/output line 87a cause resonance phenomena in each resonator 86a-86d or 89a-89h, thus functioning as a band pass filter. Therefore, only an in-band signal of the filter passes through between the input/output lines 87a and 87b and is output therefrom.
However, in the conventional high-frequency circuit element mentioned above, not only the quasi-TEM mode in which electromagnetic waves propagate through the strip conductor transmission line is excited, but also a waveguide mode is excited due to discontinuity in the converting part from the input/output terminals 84a, 84b to the input/output lines 87a, 87b, as well as due to the component of the radiation electric field caused by the quasi-TEM mode, as shown in FIG. 12. The wave guide mode is an unwanted higher-order mode, in which electromagnetic waves propagate through the space between the box lid 81 and the substrate 85. As a result, the frequency characteristic of the high-frequency circuit element is adversely affected. Similarly to FIG. 11, FIG. 12 is a cross-sectional view showing the box, which is cut along the plane perpendicular to the input/output line 87a, and illustrating the analysis result of the three-dimensional electromagnetic field simulation.
Unlike the quasi-TEM mode, since such a waveguide mode is not under the influence of the filter function, electromagnetic waves propagate in regardless of the passband of the filter, thus deteriorating the characteristic of the high-frequency circuit element as a filter. Particularly, there arise some problems, for example, the decrease in attenuation in a rejection band, i.e., the increase in the background level, the deterioration of capability of rejecting out-of-band signals of the filter.
In order to reduce the influence of the waveguide mode (unwanted higher-order mode), the box may be designed so that the cutoff frequency in the space between the box lid 81 and the substrate 85 is set to be sufficiently higher than the center frequency of the filter. However, great attenuation, for example, xe2x88x9280 dB or less is required in the rejection band and so the propagation of only a few high-frequency signals may cause a problem. Consequently, a solution only by designing the box taking into account the cutoff frequency is not sufficient.
Such a problem becomes further significant when it is necessary to miniaturize a high-frequency circuit element, to increase an operation frequency or to use a material such as a superconductive conductor having a large conductivity, in order to reduce a loss.
It is an object of the present invention to solve the problems of the prior art mentioned above. That is, the object of the present invention is to provide a high performance high-frequency circuit element capable of suppressing the generation of an unwanted higher-order mode in the space inside the metal box of the high-frequency circuit element and having an excellent frequency characteristic.
In a basic configuration, a high-frequency circuit element according to the present invention includes a substrate, a high-frequency circuit formed on the substrate, a metal box electromagnetically shielding the high-frequency circuit by enclosing the substrate, an input/output terminal placed on the metal box and inputting/outputting a high-frequency signal to/from the high-frequency circuit, and at least one shielding element for interrupting an unwanted higher-order mode by suppressing the propagation of high frequency waves between the input-output terminals.
In a first specific configuration, a high-frequency circuit element according to the present invention includes a substrate, a high-frequency circuit formed on the substrate, a metal box electromagnetically shielding the high-frequency circuit by enclosing the substrate, an input/output terminal placed on the metal box and inputting/outputting a high-frequency signal to/from the high-frequency circuit, and at least one plate for interrupting an unwanted higher-order mode substantially dividing an internal space of the metal box and cutting off the propagation path for the high-frequency waves in the internal space of the metal box.
According to such a configuration, having the plate for interrupting an unwanted higher-order mode, it is made possible to suppress the propagation of the electromagnetic waves in a waveguide mode generated in the space inside the box so as to prevent the deterioration of the frequency characteristic due to excitation of the unwanted higher-order mode. Therefore, it is possible to provide a high performance high-frequency circuit element having an excellent frequency characteristic.
It is desirable in the above-mentioned configuration that the plate for interrupting an unwanted higher-order mode is made of a conductor. Thus, the effect of suppressing the propagation of the unwanted higher order mode can be improved, because the conductor does not transmit electromagnetic waves. In such a configuration, it is desirable that the plate for interrupting an unwanted higher-order mode made of a conductor is electrically connected to the metal box. Thus, the plate for interrupting an unwanted higher-order mode made of a conductor functions as an electric wall with respect to the electromagnetic waves, so that the effect of suppressing the propagation of the unwanted higher-order mode can be enhanced further.
Furthermore, in the above-mentioned first specific configuration, it is desirable that the plate for interrupting an unwanted higher-order mode is made of a dielectric having a high dielectric constant. Thereby, electromagnetic waves are reflected from or absorbed by the dielectric and cannot pass through the plate. Consequently, the effect of suppressing the propagation of the unwanted higher-order mode can be enhanced.
It is desirable in the first specific configuration that the plate for interrupting an unwanted higher-order mode is placed spanning over and approximately perpendicular to at least one input/output line of the high-frequency circuit and placed so that it is not in an electric contact with the input/output line. Thereby, the connection portion where the input/output terminal is connected to the input/output line is isolated from the main circuit part, such as a resonator, of the high-frequency circuit. Consequently, it is possible to prevent the electromagnetic waves of the unwanted higher-order mode, which are excited due to the discontinuity of the connection portion, from adversely affecting the high-frequency circuit. It is desirable in this configuration that the plate for interrupting an unwanted higher-order mode has a cut-out so that it is not in electric contact with the input/output lines of the high-frequency circuit. Thereby, the lower end of the plate can be extended to the position that is in contact with the substrate in a place where the high-frequency circuit element is not provided. Consequently, the effect of suppressing the unwanted higher-order mode can further be enhanced.
According to the second specific configuration of the high-frequency circuit element according to the present invention, the high-frequency circuit element includes a substrate, a high-frequency circuit formed on the substrate, a metal box electromagnetically shielding the high-frequency circuit by enclosing the substrate, an input/output terminal placed on the metal box and inputting/outputting a high-frequency signal to/from the high-frequency circuit, and at least one cover for interrupting an unwanted higher-order mode covering at least one input/output line of the high-frequency circuit in an internal space of the metal box and suppressing the propagation of high-frequency waves.
According to such a configuration, having the cover for interrupting an unwanted higher-order mode makes it possible to suppress the propagation of the electromagnetic waves in a waveguide mode generated in the space inside the box so as to prevent the deterioration of the frequency characteristic due to excitation of the unwanted higher-order mode. Therefore, it is possible to provide a high performance high-frequency circuit element having an excellent frequency characteristic.
It is desirable in the second specific configuration that the cover for interrupting an unwanted higher-order mode is made of a conductor. Thus, the effect of suppressing the propagation of the unwanted higher-order mode is enhanced, because the conductor does not transmit electromagnetic waves. It is desirable in this configuration that the cover for interrupting an unwanted higher-order mode made of a conductor is electrically connected to the metal box. Thus, the cover for interrupting an unwanted higher-order mode functions as an electric wall with respect to electromagnetic waves. Consequently, it is possible further to enhance the effect of suppressing the propagation of the unwanted higher-order mode.
It is desirable in the second specific configuration that the cover for interrupting an unwanted higher-order mode is made of a dielectric having a high dielectric constant. Thus, electromagnetic waves are reflected from or absorbed by the dielectric and cannot pass through the cover, so that the effect of suppressing the propagation of the unwanted higher-order mode can be enhanced.
It is desirable in the first and second specific configurations that the high-frequency circuit is a high-frequency filter. Thus, excitation of the unwanted higher-order mode is suppressed by the plate or cover for interrupting an unwanted higher-order mode. Consequently, it is possible to prevent the higher-order mode in the out-of-band frequency, which is not rejected by the filter, from propagating and being output. Therefore, the filter having an excellent out-of-band attenuation characteristic can be attained. In this configuration, it is desirable that the high-frequency filter has a plurality of coupled planar circuit resonators. Thus, it is possible to prevent the unwanted higher-order mode excited by the radiation from the resonators from inputting from the input/output line and outputting from the input/output terminal. Consequently, it is possible to prevent the higher-order mode in the out-of-band frequency that is not rejected by the filter from propagating and being output. Therefore, it is possible to attain a filter having an excellent out-of-band attenuation characteristic.
Furthermore, in the first and second specific configurations, it is desirable that the high-frequency circuit is a superconductor filter. Thus, it is possible to radically reduce the conductor loss at the resonator, so that low loss and a steep skirt can be attained.